Devices are known in which a TV camera is used to detect a visible image produced at a screen on which is imaged an energetic beam of penetrative radiation, as aforesaid, which is at least in part transmitted through a light-opaque specimen.
For example, in the transmission electron microscope (TEM), energetic electrons are accelerated and focussed on to a specimen. Those that pass through the specimen are re-imaged on to a scintillating screen which emits light when the electrons strike it. The light signal is conventionally imaged, typically by a TV camera lens coupled to the scintillating screen. In order to achieve high resolution, the electrons in the beam need to be accelerated to high energies. As a result, those that are absorbed by the specimen transfer their considerable kinetic energy to the specimen material, causing it to heat up. In many applications the heating caused by the beam current necessary to provide an adequate image on the TV camera leads to specimen damage that not only compromises the quality of the image and the results obtained but will often destroy the specimen entirely.
The situation in X-ray imaging is very similar. An X-ray source creates a point source of X-rays which pass through the specimen. X-rays for practical purposes cannot be focussed, so that all imaging is in fact a simple shadow-image of the specimen. The shadow image falls on a sheet of X-ray sensitive film or on to a phosphor screen to create a visible image which is again detected by a TV camera suitably coupled to the image. As with the electron beam case, X-ray beam damage imposes an important limitation on the kind of specimens which may be inspected with X-rays.
U.S. Pat. No. 4,503,460 discloses an X-ray diagnostic arrangement in which X-rays passing through a specimen are received by an X-ray image intensifier. By application of a high voltage the image intensifier produces a light image on a fluorescent output screen which is amplified and intensified compared with the strength of the input image. The intensifier output is optically coupled, by means of a plurality of lenses, to semiconductor image sensors such as CCD image converters, which may be cooled e.g. by Peltier elements. The outputs of the image sensors are converted into video signals, which are read and displayed at TV frame rates, i.e. in real time. The use of an image intensifier increases signal strength but also has the effect of adding noise to the signal, so compromising the quality of the final image. Further, image intensifiers need voltages of many kilovolts to operate, are prone to damage from signal overload and have poor geometric fidelity, giving up to 30% distortion at the edge of their field of view. They also have very poor dynamic range.
It is an aim of the present invention to provide a method and apparatus for imaging light opaque specimens by transmission of penetrative radiation therethrough capable of giving better results than have been possible hitherto.